29 1 MISCELLANEOUS OBJECTIONS TO THE [Chap. VII. 



of their disuse, will be strengthened by natural selection. 

 For all spontaneous variations in the right direction 

 will thus be preserved ; as will those individuals which 

 inherit in the highest degree the effects of the increased 

 and beneficial use of any part. How much to attribute 

 in each particular case to the effects of use, and how 

 much to natural selection, it seems impossible to decide. 

 I mav d-ve another instance of a structure which 

 apparently owes its origin exclusively to use or habit. 

 The extremity of the tail in some American monkeys 

 has been converted into a wonderfully perfect prehensile 

 organ, and serves as a fifth hand. A reviewer who 

 agrees with Mr. Mivart in every detail, remarks on this 

 structure : " It is impossible to believe that in any 

 number of ages the first slight incipient tendency to grasp 

 could preserve the lives of the individuals possessing it, 

 or favour their chance of having and of rearing offspring." 

 But there is no necessity for any such belief. Habit, 

 and this almost implies that some benefit great or small 

 is thus derived, would in all probability suffice for the 

 work. Brehin saw the young of an African monkey 

 (Cercopithecus) clinging to the under surface of their 

 mother by their hands, and at the same time they hooked 

 their little tails round that of their mother. Professor 

 Henslow kept in confinement some harvest mice (Mus 

 rnessorius) which do not possess a structurally pre- 

 hensile tail ; but he frequently observed that they curled 

 their tails round the branches of a bush placed in the 

 cage, and thus aided themselves in climbing. I have 

 received an analogous account from Dr. Gunther, who 

 has seen a mouse thus suspend itself. If the harvest 

 mouse had been more strictly arboreal, it would perhaps 

 have had its tail rendered structurally prehensile, as is 



